Field-induced nematic-like magnetic transition in an iron pnictide superconductor, Ca$_{10}$(Pt$_{3}$As$_{8}$)((Fe$_{1-x}$Pt$_{x}$)$_{2}$As$_{2}$)$_{5}$

We report a high magnetic field study up to 55~T of the nearly optimally doped iron-pnictide superconductor Ca$_{10}$(Pt$_{3}$As$_{8}$)((Fe$_{1-x}$Pt$_{x}$)$_{2}$As$_{2}$)$_{5}$ with a $T_{c} \approx$ 10 K using magnetic torque, tunnel diode oscillator technique and transport measurements. We determine the superconducting phase diagram, revealing an anisotropy of the irreversibility field up to a factor of 10 near $T_{c}$ and signatures of multiband superconductivity. Unexpectedly, we find a spin-flop like anomaly in magnetic torque at 22~T, when the magnetic field is applied perpendicular to the {\it ab} planes, which becomes significantly more pronounced as the temperature is lowered 0.33~K. As our superconducting sample lies well outside the antiferromagnetic region of the phase diagram, the observed field-induced transition in torque indicates a spin-flop transition \textit{not of long-range ordered moments}, but of nematic antiferromagnetic fluctuations. This work was supported by the EPSRC (EP/I004475/1) and EuroMagNET II.